# std::scoped_allocator_adaptor<OuterAlloc,InnerAlloc...>::construct

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< cpp‎ | memory‎ | scoped allocator adaptor

Defined in header `<scoped_allocator>` |  |
template< class T, class... Args > void construct( T* p, Args&&... args ); | (1) |
template< class T1, class T2, class... Args1, class... Args2 > void construct( std::pair<T1, T2>* p, std::piecewise_construct_t, std::tuple<Args1...> x, std::tuple<Args2...> y ); | (2) | (until C++20)
template< class T1, class T2 > void construct( std::pair<T1, T2>* p ); | (3) | (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, U&& x, V&& y ); | (4) | (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy ); | (5) | (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy ); | (6) | (until C++20)
Helper function templates |  |
template < class T, class... Args > std::tuple</* see below */> /*concat-args*/( std::tuple<Args...>&& tup ); | (7) | (exposition only*) (until C++20)

Constructs an object in allocated, but not initialized storage pointed to by p using the outer allocator and the provided constructor arguments. If the object is of a type that itself uses allocators, or if it is std::pair(until C++20), passes the inner allocator down to the constructed object.

1) Constructs an object of type `T` by uses-allocator construction at the uninitialized memory location indicated by p using the outermost allocator.

Given std::uses_allocator<T, inner_allocator_type>::value as uses_inner: If uses_inner is false and std::is_constructible<T, Args...>::value is true, calls `outermost-construct`﻿(p, std::forward<Args>(args)...). Otherwise, if uses_inner and std::is_constructible<T, std::allocator_arg_t, inner_allocator_type&, Args...>::value are both true, calls `outermost-construct`﻿(p, std::allocator_arg, `` inner_allocator(), `` std::forward<Args>(args)...). Otherwise, if uses_inner and std::is_constructible<T, Args..., inner_allocator_type&>::value are both true, calls `outermost-construct`﻿(p, std::forward<Args>(args)..., inner_allocator()). Otherwise, the program is ill-formed. This overload participates in overload resolution only if `T` is not a specialization of std::pair. | (until C++20)
Equivalent to std::apply ( `` [p, this](auto&&... newargs) `` { `` `outermost-construct` `` (p, std::forward<decltype(newargs)>(newargs)...); `` }, `` std::uses_allocator_construction_args `` (inner_allocator(), std::forward<Args>(args)...) ); . | (since C++20)

2-6) Constructs a std::pair object by uses-allocator construction at the uninitialized memory location indicated by p using the outermost allocator.
2) Let xprime be `concat-args`﻿<T1>(std::move(x)), yprime be `concat-args`﻿<T2>(std::move(y)), calls `outermost-construct`﻿(p, std::piecewise_construct, std::move(xprime), std::move(yprime)).
3) Equivalent to construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>());.
4-6) Equivalent to construct(p, std::piecewise_construct,
std::forward_as_tuple(xarg), std::forward_as_tuple(yarg));, where xarg and yarg are defined as follows:

Overload | xarg | yarg
--- | --- | ---
(4) | std::forward<U>(x) | std::forward<V>(y)
(5) | xy.first | xy.second
(6) | std::forward<U>(xy.first) | std::forward<V>(xy.second)

7) Merges the arguments contained in tup and additional arguments required by uses-allocator construction of an object of type `T`.
Given std::uses_allocator<T, inner_allocator_type>::value as uses_inner:

- If uses_inner is false and std::is_constructible<T, Args...>::value is true, returns std::tuple<Args&&...>(std::move(tup)).
- Otherwise, if uses_inner and std::is_constructible<T, std::allocator_arg_t,
  inner_allocator_type&,
  Args...>::value are both true, returns std::tuple_cat(std::tuple<std::allocator_arg_t, inner_allocator_type&>
  (std::allocator_arg, inner_allocator()),
  std::tuple<Args&&...>(std::move(tup))).
- Otherwise, if uses_inner and std::is_constructible<T, Args..., inner_allocator_type&>::value are both true, returns std::tuple_cat(std::tuple<Args&&...>(std::move(tup)),
  std::tuple<inner_allocator_type&>(inner_allocator()).
- Otherwise, the program is ill-formed.

### Parameters

p | - | pointer to allocated, but not initialized storage
args | - | the constructor arguments to pass to the constructor of `T`
x | - | the constructor arguments to pass to the constructor of `T1`
y | - | the constructor arguments to pass to the constructor of `T2`
xy | - | the pair whose two members are the constructor arguments for `T1` and `T2`
tup | - | the arguments to be merged

### Notes

This function is called (through std::allocator_traits) by any allocator-aware object, such as std::vector, that was given a std::scoped_allocator_adaptor as the allocator to use. Since `inner_allocator_type` is itself a specialization of std::scoped_allocator_adaptor, this function will also be called when the allocator-aware objects constructed through this function start constructing their own members.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2203 | C++11 | inner allocators were obtained by value-initializing an `inner_allocator_type` object | obtained by calling `inner_allocator()`
LWG 2511 (P0475R1) | C++11 | `concat-args` might copy elements of std::tuples | eliminated all element copy operations
LWG 2586 | C++11 | only constructions from `inner_allocator_type` rvalues were checked | checks constructions from non-const `inner_allocator_type` lvalues instead
LWG 2975 | C++11 | overload (1) was not constrained | constrained to refuse std::pair

### See also

construct [static] | constructs an object in the allocated storage (function template)
construct (until C++20) | constructs an object in allocated storage (public member function of `std::allocator<T>`)

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